This tool draws the Sun's own grid of latitude and longitude on a white-light photograph you took, turned to the angle the Sun stood at the moment the shutter opened. Open the photo: it finds the disc, reads the capture time out of the file, works out the tilt of the solar axis and the angle it makes with your camera, and draws the grid. Read a sunspot's heliographic latitude and longitude straight off it, follow a group from day to day, or check whether a spot sits where the solar cycle says spots belong.
How to use it
- Open a white-light photo of the Sun: a JPEG, PNG or TIFF, taken through a solar filter. Nothing is uploaded — the file is read in your browser and goes no further.
- Check the capture time. It is read out of the file where the camera wrote one, with the UTC offset beside it. Where the file is silent, type the date and time and say which offset from UTC they are in: the whole answer turns on this.
- Say how the camera was mounted. On a tripod or an alt-azimuth mount with the camera level, enter where you stood, because a level camera is square to the horizon and not to the pole. On an equatorial mount with celestial north up, nothing more is needed.
- Check the disc. The tool measures the limb and fits a circle to it, and says how closely it fitted. Where a cloud or a cropped edge defeats it, type the centre and the radius in pixels.
- Turn the camera rotation until the grid sits square on the disc. The quarter-turn buttons cover a camera mounted sideways; the mirrored switch covers a star diagonal, which swaps east and west.
- Set the grid to taste: the spacing, which opens at 15°, one hour of the Sun's rotation; the line colour, width and opacity; the degree labels; and the solar active zone.
- Save the picture as a PNG at the photo's own resolution, with the date and the tilt written along the foot.
How it works
Two angles place the grid. The position angle P is how far the Sun's north pole leans east of celestial north, and it swings between about −26° and +26° over a year. The tilt B0 is the heliographic latitude at the centre of the disc, between −7.25° and +7.25°, which is why you see over the Sun's north pole in September and over its south pole in March. Both follow from the Sun's apparent longitude and the known tilt of the solar equator.
What those angles are measured from depends on the camera. An equatorial mount holds celestial north up, so the grid is turned by P alone. A camera level on a tripod is square to the horizon instead, and celestial north leans from the vertical by the parallactic angle, which depends on where you stood and the hour — so the grid is turned by P less that angle. Your own rotation is added on top.
The grid itself is the sphere seen flat. A point at heliographic latitude and longitude is turned by B0 and projected onto the disc; the half facing away from Earth is dropped, and each line is cut at the limb. Longitude is measured from the central meridian, positive toward solar west, which is the Stonyhurst convention — the same lines a sunspot's position is quoted in.
The solar active zone comes from the date, not from a setting. Sunspots emerge near 28° of latitude as a cycle opens and drift toward the equator through it, reaching about 7° by its end: Spörer's law. The tool reads which cycle the photo falls in, counts the months from that cycle's minimum, and draws the band in both hemispheres.
Formula
λ(t) = 28° × exp(−t ÷ 90)
λ is the latitude sunspots are forming at, in degrees, north and south alike. t is the months since the sunspot cycle began. The band is drawn two RMS widths either side of λ, taking the 5.3° Hathaway fits for a cycle in spate.
Worked example
A photo taken on 2026 June 25 falls in cycle 25, whose minimum was 2019 December, so t is about 78 months. That gives λ = 28° × exp(−78 ÷ 90) = 11.8°, and a band from 1.2° to 22.4° in each hemisphere. A spot at 10° north sits inside it; one at 35° would be worth checking.
Common questions
What is a Stonyhurst grid?
It is the Sun's own latitude and longitude drawn as they appear from Earth, with longitude counted from the central meridian rather than from a fixed point on the Sun. It is how a sunspot's position is quoted, and it is named for the observatory whose printed discs observers once laid over their drawings.
Why does the grid need the time the photo was taken?
Because both angles that place it change. The tilt of the solar axis toward Earth runs through its whole range over a year, and the angle the axis makes with your camera changes through the day as well, on a camera that is level rather than tracking. A time an hour out can turn the grid by several degrees.
The grid is turned the wrong way. What do I change?
Check the UTC offset first, then whether the image is mirrored — a star diagonal swaps east and west, and the grid will look reflected rather than rotated. If it is simply turned, use the quarter-turn buttons and the rotation. With tracking off, the Sun drifts toward celestial west, which tells you which way round the frame is.
What does the solar active zone show?
The band of latitude sunspots are emerging in on the date of the photo, worked out by Spörer's law rather than asked for. Spots appear at high latitudes early in a cycle and close on the equator through it, so the band tells you whether what you photographed is where the cycle says it should be.
How accurate is the position I read off the grid?
Near the centre of the disc, about a degree, limited by how well the limb was measured and how square the grid was turned. Toward the limb it is much worse: longitude is foreshortened there, so a small error in the drawn position becomes a large one in degrees. The active zone is looser still — a cycle's start is known to within a few months, which is a degree or two of latitude.
Is it safe to photograph the Sun?
Only through a proper solar filter fitted over the front of the telescope, checked for pinholes before every use. Never look at or photograph the Sun without one, and never use a filter that screws into an eyepiece. This tool reads a photo you already have.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: